Gain control circuits



April 11, 1939.

R. W. CHESNUT GAIN CONTROL CIRCUITS Filed Aug. 5, 1957 u Vvv/l /NVENTOR RW CHESNUT A TTORNE'V Patented Apr. 11, 1939 UNITED STATES PATENT OFFICE v2,154,062 GAIN CONTROL CIRCUITS Application August 5, 1937, serial No.' 157,498

16 Claims.

This invention relates to gain control circuits for signal transmission lines and particularly to gain control circuits controlled by pilot currents on the transmission lines.

One object of the invention is to provide gain control circuits for a transmission line having a resistance element with a high temperature coefficient of resistance for controlling a repeater amplier that shall continuously control the resistance element in an improved manner to maintain the attenuation of the transmission line constant.

Another object of the invention is to provide gain control circuits for a transmission line having a resistance element With a high temperature coefcient of resistance in an oven for con` trolling a repeater amplier that shall controlV the oven temperature according to the energy level of the currents on the transmission line beyond the amplier and that shall compensate the oven temperature for changes in the room temperature. Y

A further object of the invention is to provide gain control circuits for a transmission line having a'resistance element of silver sulphide in an oven for controlling a repeater amplier that shail control the oven temperature according to the energy level of a pilot current on the transmission line to hold the line attenuation constant 3.9:.. and that shall control the oven temperature according to the room temperature to compensate the oven temperature for changes in the room temperature.

The attenuation of a long signal transmission line, varies with changes in the Weather and changes intemperature. In order to effect good quality of signal transmission it is necessary to hold the attenuation of a signal transmission line 40 substantially constant. The attenuation of a transmission line may be controlled either by controlling an attenuation network on the line or by controlling a repeated amplier on the line. 45..., The invention has been illustrated by means 4". of a carrier current telephone system employing one range of carrier currents for transmitting signals in one direction and another range of carrier currents for transmitting signals in an 5,9; opposite direction. A pilot current is employed with each range of carrier currents for gain control purposes. `At repeater stations the east-to- West currents are separated from the west-toeast Ycurrents by means vof directional filters. The repeater amplifiers in the transmission paths at a repeater station are provided with feed-back circuits which areV controlled to maintain the attenuation in each transmission path substantially constant. i

According to the invention a resistance element having a hightemperature coeicient of resistance is included in an attenuation network. The attenuation networl:` is included in the feedback circuit of a repeater amplier. The resistance element is preferably composed of silver sulphide but may be composed of barium, zinc oxide or any other suitable material having a high temperature. coeilicient of resistance if so desired. Anoven .or enclosure which encloses the element of silver sulphide also encloses two heaters so that the silver sulphide and the two heaters are substantially at the same temperature. One of the heaters controls the oven temperature according to the `energy level of the pilot current on the transmission path beyond the repeater amplifier. The second heater is energized according to changes in the room temperature to compensate the oven temperature for changes in the room temperature. The oven or enclosure may be omitted if the silver sulphide and two heaters are in close proximity to each other.

The rst heater comprises resistance elements having different temperature coeflicients of resistance which are connected in series and parallel circuit relation. This heater may be com- 35 posed of silver sulphide elements having a high negative temperature coeflicient of resistance and resistance elements of iron which have small resistance variations for the temperatures emo ployed. i The elements comprising the heater are designed so that the heater will have a predetermined temperature resistance characteristic for controlling the silver sulphide element in the attenuation ynetwork to maintain the attenuation of the transmission path constant. A variable resistance element connected in series with the rst heater andy-located outside the oven may be ,adjusted to effect under regulation, perfect regulation or over regulation. A lter which selects the pilot current on the transmission path is connected to the path beyond the repeater amplier therein. The output from the filter is arnplied and supplied to the first heater for controlling the oven temperature.

The second heater which may be composed of any suitable resistance material having substantially no resistance variation for the temperatures employed is connected to a suitable energizing circuit outside the oven. In the energizing circuit are connected resistance elements having different temperature coeflicients of resistance. The resistance elements in the energizing circuit outside the oven may be composed of silver sulphide and iron and are subject to room temperature. The energizing circuit for the secof alternating current.

In the accompanying drawing, Fig. l is a diagrammatic view of gain control circuits constructed in accordance With the invention;

Fig. 2 is a set of curves illustrating the resistance temperature characteristic oftlie control heater shown in Fig. 1; and Y,

Fig. 3 is a set of curves showing how the output level is regulated.

Referring to Fig. 1 of the drawing, a repeater station having a west-to-east channel or path 2, and an east-to-west path 3 is shown on a transmission line comprising east conductors 4 and 5 and west conductors 6 and 1. The west-to-east channel 2 comprises a directional filter 8, an equalizing network 9, a repeater amplifier I which is preferably of the thermionic type, and a directional filter I I. The directional lters 8 and I I select the carrier currents'which are employed to transmit signals from the west to the east. The directional filters 8 and 4IIl also select the pilot current employed to control the gain` of the carrier currents employed for transmitting signals from the west to the east.

The east-to-west channel or path 3 comprises a directional lter I2, an equalizing network I3,

Vga repeater amplifier I4, and a directional filter The directional lters I2 and I5 select the carrier currents employed-to transmit signals from the east Vto the west. The directional filters I2 and I5 also select the pilot current employed for controlling the amplifier I4. Inasmuch as lthe two channels 2 and 3 operate in a similar manner, a detailed description will be given only of the control of the channel 3 for maintaining the`line attenuation constant.

The repeater amplifier I4 comprises a space discharge device I6 having an anode I1, a cathode I8, a grid I9 and a screen grid 20. The input circuit for the space discharge device I6 is connected to the channel 3 by means of a hybrid The output circuit for the device II is connected to the channel 3 by means of a hybrid coil network 22. 23 is provided for the grid I9 and battery 24 is provided for the anode I1.

A feedback circuit 25 for controlling the space discharge device I6 is connected between the hybrid coil network 2| and the hybrid coil network 22. The feedback circuit comprises a regulating network 26 and a fixed equalizer 21. The regulating network 26 is controlled in a manner to be hereinafter set forth for controlling the feedback on the amplier I6 to maintain a transmission line attenuation on the path 3 substantially constant. A repeater amplifier of the type above described is disclosed in the patent to A biasing battery H. S. Black, No. 2,102,671, December 21, 1937. Reference may be had to this application for a more detailed description of the operation of the amplifier.

The regulating network 26 comprises resistance elements 28, 29, 30 and 3|, a capacity 32, an inductance 33, and a resistance element 34 having a high temperature coefficient of resistance. Preferably the resistance element 34 is composed of silver sulphide. It is to be understood that the resistance element 34 may be composed of other materials, such as barium or zinc oxide ii so desired. The element of silver sulphide 34 is positioned inside an oven or enclosure 35. The oven 35 also encloses two heaters 36 and 31.

`The heater 36 is energized according to the energy level of the pilot current on the channel 3 beyond the repeater amplifier I6. The heater 31 is energized according to variations in room temperature to compensate the oven temperature for changes in room temperature. The heater 36 comprises two elements 38 and 39 having high temperature coefficients of resistance and three resistance elements 40, 4I and 42, which have a different temperature coefficient of resistance. Preferably the elements 38 and 39 are composed of silver sulphide. The elements 40, 4I and 42 may be composed of iron resistance and should have little change in resistance for the temperature range employed in the oven. The heater 31 comprises the resistance element 43 which has a substantially constant resistance within the working range of temperatures employed in the oven.

A lter 44 comprising two transformers 45 and 46 and two adjustable condensers 41 and 48 is connected to the channel 3 beyond the repeater amplifier I6 for selecting the pilot current on the channel 3. The lter 44 is connected to the input circuit of a space discharge amplifier 49. The output circuit of the amplifier device 49 is connected by a transformer 50 to an energizing circuit for the heater 36. An adjustable resistance element I is included in the energizing circuit for the heater 36. The resistance element 5I is positioned outside the oven 35 for determining whether the regulation effected by the silver sulphide element 34 will be over regulation, perfect regulation, or under regulation, as the level coming in from line 4, 5 rises and falls due to weather change.

An energizing circuit 52 for the heater 31 is connected by a transformer 53 to a source of alternating current 54. A resistance network 1I is connected in the energizing circuit 52 for controlling the energization of the heater 31 according to changes in the room temperature. The network 1I comprises an element 55 of silver sulphide having a high temperature coefficient of resistance and two resistance elements 56 and 51, which may be common iron resistance. The network 1I is adjusted so that the heater 31 compensates for the loss in temperature from the oven 35 eiected by changes in room temperature.-

In the circuit employed to describe the invention, the oven 35 has a working temperature range between 120 and 350 Fahrenheit. When working the oven 35 within the above-mentioned temperature range approximately all loss of heat from the oven will be by conduction. Radiation of heat from the oven is negligible. The rate of heat loss will ythen be closely proportional to the H :heat which escapes from oven into the room per second.

' k :heat loss constant (from oven to room).

=heat put into oven by heater 31 per second. :resistance of heater 36 which is a function of t. Ro:resistance 5I plus the plate resistance of the tube 49 as seen through transformer 50.

es :potential supplied by transformer 50 which is proportional to the pilot level put out by amplier I4.

Then, when the temperature of the oven 35 has reached a stable state;

The above Equation 2 may be written as follows:

H:k[(t120)+(120-T)l (3) In Equation 3 it is apparent that we can make I h:1c(t-120) and h=k(120-T) (4) This makes h a function of oven temperature only and h. a function of room temperature only.

From Equation 6 the value of `an ideal resistance R as a function of the temperature t can be determined in terms of es. Ro and lc, which are known parameters.

Referring to Fig. 2 ofthe drawing, the curve a having the abscissae in terms of t", the temperature within the oven,` and the ordinates in terms of resistance on a logarithmic scale represents the ideal resistance R for the heater 36. The curve f represents a practical design. 'I'he working range for which a practical R can be designed would be between the points b and c on the curve a where the practical resistance closely coincides with the ideal. The curve d shows the resistance R for the heater if formed of silver sulphide alone.

In the above equations it has been assumed that the room temperature is constant at 120 for convenience in explaining the operation of the regulator. The changes in room temperature which may take place are compensated for by means of the heater 31. Thecurve a in Fig. 2 of the drawing shows the resistance temperature characteristic which the heater 36 must haveto maintain the attenuation on the east-to-west channel of the transmission lineconstant. The heater 36 as before set forth is composed of resistance elements having different temperature coefficients of resistance so combined as to approximate the ideal over the working range bv to c in Fig. 2. d K

If the input energy level of the pilot current on the channel 3 tends to rise above normal, the voltage e3 which is supplied by the transformer 50 to the energizing circuit for the heater 36 is raised. The temperature i of the heater 36 is raised for controlling the silversulphide element.

34. The temperature resistance characteristic of the heater 36 follows the curve a shown in Fig, 2 of the drawing. The increase in temperature lowers the resistance of the silver sulphide element 34. The lowering of the resistance of the element 34 lowers the impedance of the network 26 in the feedback circuit 25. This causes an increased feedback to lower the gain of amplifier I4, and therefore lower the energy level of the currents put out on the channel 3.

yIn case the energy level of the pilot current on the channel 3 beyond the amplifier I6 is lowered below normal level, then the potential supplied by the transformer 50 is reduced. This causes a reduction'in the heat supplied by the heater 36 to the silver sulphide element 34. A reduction in the temperature of the silver sulphide element 34 increases the resistance of the controlling network 26 to lower the negative feedback on the amplifier I6. By such means the energy level of the signal currents on the channel 3 is raised.

The adjustable resistance 5I in the energizing circuit is one me'ans for adjusting R0 to obtain the desired regulating characteristic for the amplifier I4. When Ro is made just the right value the regulator will always bring the input level from amplifier I4 back to the same constant level p in Fig. 3 each time the input level to amplifier I4 is changed up or down; that is, we get perfect regulation as shown by curve n in Fig. 3. When R0 is made larger than this value the regulator will not bring the output level all the way back to its original level when the input level is changed up or down; that is, we get under regulation as shown in curve m. On the other hand, when R0 is made smaller than this value the regulator will over regulate; that is, a higher input level to amplifier I4 will bring about a lower output level and vice versa as shown in curve o. The amount of under regulation or over regulation desired may be adjusted by adjusting R0. It is obvious that other means than resistance 5I may be used to vary Ro such for instance as taps on transformer 50.

The resistance network 1I in the energizing circuit for the heater 31 is subjected to room temperature changes and controls the heater 31 in accordance with changes in the room temperature. 'Ihe resistance elements having different temperature coefficients of resistance are varied to insure compensating the oven temperature for variation in the room temperature.

The channel 2 operates in exactly the same manner as described above with respect to channel 3. Gain control circuits 10 are provided for controlling the repeater amplifier IB in the same manner as the repeater amplifier in channel 3 is controlled.

Modications in the circuits and in the arrangement and location of parts may be made within the spirit and scope of the invention and such modifications are intended to be covered by the appended claims.

What is claimed is:

l. In a regulator for controlling a characteristic of a circuit, an oven, means comprising a control element in said oven having a high temperature coeiicient of resistance for controlling said characteristic of the circuit, a heater in said oven having a regulating characteristic and comprising resistance elements arranged in seriesv and parallel circuit relation, the resistance elements of said heater having different rates of resistance variations with temperature changes for obtaining a complex heater that variesin resistance with temperature changes to effect a regulating operation and means controlled according to the characteristic of said circuit for energizing,

said heater to heat said control element according to the heater characteristic and maintain the control element in said oven having a high temperature coeilicient of resistance for controlling said characteristic of the circuit, means comprising a heater in said oven to control the temperature of said element, according to changes in said characteristic, a second heater in said oven, a

- source of energy in an energizing circuit for said second heater, and means comprising resistance elements subjected to room temperature, having different temperature coeilicients of resistance and inserted in said energizing circuit to compensate the oven temperature for changes in the room temperature.

3. In a gain control circuit for a signal transmission line, an oven, means comprising a control element in said oven having a high temperature coeicient of resistance for controlling the transmission line attenuation, a heaterl for said control element in said oven comprising resistance eleand parallel circuit relation and having different temperature coeilicients of resistance for controlling said element to maintain the line attenuation constant, means for energizing said heater according to the energy level of the signals on the transmission line, a second heater in said oven, and means for energizing said second heater to compensate the oven temperature for changes in room temperature.

5. In a gain control circuit for a signal transmission line, an oven, means comprising a control element in said oven having a high temperature coeilicient of resistance for controlling the transvmission line attenuation, a heater in said oven for said control element comprising resistance elements arranged in series and parallel circuit revlation and having different temperature coeilicients of resistance, means for controlling said heater according to the line attenuation, a second heater in said oven, and means controlling said second heater comprising a source of energy and resistance elements subjected to room ternperature and having different temperature coecients of resistance to compensate the oven temperature for changes in the room temperature.

6. In gain control circuits for a signal transmission line, an oven, means comprising a control element in said oven having a high temperature coeiilcient of resistance for controlling the transmission line attenuation, a heaterv in said oven comprising resistance elements arranged in series and parallel circuit relation and having diiierent temperature coeicients of resistance for producing a temperature resistance characteristic of a predetermined character, a variable resistance in the energizing circuit of said heater to selectively effect flat gain, raising gain and lowering gain by the control circuits, and means for controlling said heater according lto the energy level of the signalson the transmission line.

7. In gain control circuits for a signal transmission line, an oven, a control element of silver sulphide in said oven, means for contro-lling the transmission line attenuation according to the temperature of the element, a heater in said oven comprising elements of silver sulphide connected in series and shunt circuit relation to resistance elements having a different temperature coefficient oi resistance, meansI for energizing said heater according to the energy level of the signais on the transmission line to govern said control element of silver sulphide and maintain the line attenuation constant, a second heater in` said oven, a source of current in an energizing circuit for said second heater and means comprising resistance elements in said energizing circuit for controlling said second heater to compensate the o-ven temperature for changes in room temperature, said last-mentioned resistance elements having different temperature coeflicients of resistance and are subjected to room temperature.

8. In gain control circuits for'maintaining the attenuation on a transmission` line constant, an oven, an element of silver sulphide in said oven for contro-lling the line attenuation according to the temperature thereof, a heater in said oven comprising resistance elements having diierent temperature coefficients of resistance, means for energizing said heater according to the energy level of the signals on the transmission line to control the temperature of said silver sulphide and maintain the line attenuation constant, a second heater in said oven, and means comprising a source of current and a resistance arrangement subjected to room temperature for controlling the energization of said second heater to compensate the ovenxtemperature for changes in the room temperature.

9. In combination, a transmission line carrying signal currents, an oven enclosing a resistance element having a high temperature coeilicient of resistance, means controlled by said resistance element for controlling the attenuation of the transmission line, a heater in said oven for controlling sad resistance element, means for energizing said heater according to the energy level of the currents on said lin-e, a second heater in said oven, and means for energizing said second heater to compensate the oven temperature for changes in the room temperature.

10. In combination, a transmission line carrying signal currents, an oven, a resistance element in said oven having a high temperature coeilcient of resistance for controlling the attenuation of said line at a predetermined control point, a heater in said oven for controlling the temperature of said resistance element, means for energizing said heater according to the energy level of the signalcurrents beyond said control point to maintain the line attenuation constant, a second heater in the oven, and means for energizing said second heater to compensate the oven temperature for changes in the room temperature.

11. In combination, a transmission line carrying signal currents and pilot current, an oven, means comprising a resistance element of silver sulphide in'said oven forcontrolling the attenuation of said line at a predetermined control point, a heater in said oven for controlling the temperature of said element of silver sulphide, means for energizing said heater according to the energy level of the pilot current beyond said control point to maintain the line attenuation constant, a second heater in the oven, and means for energizing said second heater to compensate the oven temperature for changes in the room temperature.

12. In combination, a transmission line carrying signal currents 'and having a repeater amplifier therein, a feedback circuit for efl'ecting control of said amplifier, an attenuation network comprising a resistance element having a high temperature coeiicient of resistance in said feedback circuit, an oven enclosing said resistance element, a heater having a regulating characteristic for said resistance element included in said oven, said heater changing in resistance with. temperature changes for effecting a regulating operation, and means for continuously operating said heater according to the energy level of the signals on the transmission line to control said resistance element in accordance with the heater characteristic and maintain the energy level of the signal current beyond said amplifier constant.

13. In combination, a transmission line carrying signal currents and a pilot current, a repeater ampliler on said line, a feedback circuit for controlling said amplifier, an attenuation network in said feedback circuit comprising a resistance element having a high temperature coefficient of resistance, an oven enclosing said resistance element, a heater having a regulating characteristic and included in said oven for heating said resistance element, said heater changing in resistance with temperature changes for effecting a regulating operation, meansv comprising a filter for selecting said pilot current from the line beyond said amplifier, and means governed by the output from said filter for continuously energizing said heater to heat said resistance element according to the heater characteristic and maintain the energy level of the signal currents beyond said amplifier constant.

14. In combination, a transmission line carrying signal currents and a pilot current, a repeater amplifier on said line, a feedback circuit for controlling said amplifier, an attenuation network in said feedback circuit comprising an element of silver sulphide, an oven enclosing said element of silver sulphide, a heater included in said oven for heating said element of silver sulphide, means governed by the energy level of the pilot current on the line beyond said amplifier for continuously energizing said heater to maintain the energy level of the signal current beyond said amplifier constant, a second heater in said oven, and means for energizing 'said second heater to compensate for changes in the room temperature.

15. In a regulator for controlling the characteristic of a circuit, means comprising a controlelement having a high temperature coeicient of resistance for controlling said characteristic of the circuit, a heater in close proximity to said control element and having a regulating characteristic, said heater comprising resistance elements arranged in series and parallel circuit relation and having different rates of resistance variations with temperature changes for obtaining a complex heater that varies in resistance with temperature changes to eifect a regulating operation and means controlled according to the characteristic of said circuit for energizing said heater.

16. In a regulator for controlling a characteristic of a circuit, means comprising a control element having a high temperature coefcient of resistance for controlling said characteristic of the circuit, means comprising a heater in close proximity to said control element to control the temperature of the element according to changes in said characteristic, a second heater in close proximity to said control element, a source of energy in an energizing circuit for said second heater, and means comprising resistance elements subjected to room temperature, having different temperature coeiiicients of resistance and inserted in said energizing circuit to compensate for changes in the room temperature.

,ROY W. CHESNUT. 

